Messages in this thread | | | Date | Thu, 12 Jul 2018 20:58:42 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Consolidating RCU-bh, RCU-preempt, and RCU-sched |
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On Fri, Jul 13, 2018 at 11:47:18AM +0800, Lai Jiangshan wrote: > On Fri, Jul 13, 2018 at 8:02 AM, Paul E. McKenney > <paulmck@linux.vnet.ibm.com> wrote: > > Hello! > > > > I now have a semi-reasonable prototype of changes consolidating the > > RCU-bh, RCU-preempt, and RCU-sched update-side APIs in my -rcu tree. > > There are likely still bugs to be fixed and probably other issues as well, > > but a prototype does exist. > > > > Assuming continued good rcutorture results and no objections, I am > > thinking in terms of this timeline: > > > > o Preparatory work and cleanups are slated for the v4.19 merge window. > > > > o The actual consolidation and post-consolidation cleanup is slated > > for the merge window after v4.19 (v5.0?). These cleanups include > > the replacements called out below within the RCU implementation > > itself (but excluding kernel/rcu/sync.c, see question below). > > > > o Replacement of now-obsolete update APIs is slated for the second > > merge window after v4.19 (v5.1?). The replacements are currently > > expected to be as follows: > > > > synchronize_rcu_bh() -> synchronize_rcu() > > synchronize_rcu_bh_expedited() -> synchronize_rcu_expedited() > > call_rcu_bh() -> call_rcu() > > rcu_barrier_bh() -> rcu_barrier() > > synchronize_sched() -> synchronize_rcu() > > synchronize_sched_expedited() -> synchronize_rcu_expedited() > > call_rcu_sched() -> call_rcu() > > rcu_barrier_sched() -> rcu_barrier() > > get_state_synchronize_sched() -> get_state_synchronize_rcu() > > cond_synchronize_sched() -> cond_synchronize_rcu() > > synchronize_rcu_mult() -> synchronize_rcu() > > > > I have done light testing of these replacements with good results. > > > > Any objections to this timeline? > > > > I also have some questions on the ultimate end point. I have default > > choices, which I will likely take if there is no discussion. > > > > o > > Currently, I am thinking in terms of keeping the per-flavor > > read-side functions. For example, rcu_read_lock_bh() would > > continue to disable softirq, and would also continue to tell > > lockdep about the RCU-bh read-side critical section. However, > > synchronize_rcu() will wait for all flavors of read-side critical > > sections, including those introduced by (say) preempt_disable(), > > so there will no longer be any possibility of mismatching (say) > > RCU-bh readers with RCU-sched updaters. > > > > I could imagine other ways of handling this, including: > > > > a. Eliminate rcu_read_lock_bh() in favor of > > local_bh_disable() and so on. Rely on lockdep > > instrumentation of these other functions to identify RCU > > readers, introducing such instrumentation as needed. I am > > not a fan of this approach because of the large number of > > places in the Linux kernel where interrupts, preemption, > > and softirqs are enabled or disabled "behind the scenes". > > > > b. Eliminate rcu_read_lock_bh() in favor of rcu_read_lock(), > > and required callers to also disable softirqs, preemption, > > or whatever as needed. I am not a fan of this approach > > because it seems a lot less convenient to users of RCU-bh > > and RCU-sched. > > > > At the moment, I therefore favor keeping the RCU-bh and RCU-sched > > read-side APIs. But are there better approaches? > > Hello, Paul > > Since local_bh_disable() will be guaranteed to be protected by RCU > and more general. I'm afraid it will be preferred over > rcu_read_lock_bh() which will be gradually being phased out. > > In other words, keeping the RCU-bh read-side APIs will be a slower > version of the option A. So will the same approach for the RCU-sched. > But it'll still be better than the hurrying option A, IMHO.
I am OK with the read-side RCU-bh and RCU-sched interfaces going away, it is just that I am not willing to put all that much effort into it myself. ;-)
Unless there is a good reason for me to hurry it along, of course.
Thanx, Paul
> Thanks, > Lai > > > > > o How should kernel/rcu/sync.c be handled? Here are some > > possibilities: > > > > a. Leave the full gp_ops[] array and simply translate > > the obsolete update-side functions to their RCU > > equivalents. > > > > b. Leave the current gp_ops[] array, but only have > > the RCU_SYNC entry. The __INIT_HELD field would > > be set to a function that was OK with being in an > > RCU read-side critical section, an interrupt-disabled > > section, etc. > > > > This allows for possible addition of SRCU functionality. > > It is also a trivial change. Note that the sole user > > of sync.c uses RCU_SCHED_SYNC, and this would need to > > be changed to RCU_SYNC. > > > > But is it likely that we will ever add SRCU? > > > > c. Eliminate that gp_ops[] array, hard-coding the function > > pointers into their call sites. > > > > I don't really have a preference. Left to myself, I will be lazy > > and take option #a. Are there better approaches? > > > > o Currently, if a lock related to the scheduler's rq or pi locks is > > held across rcu_read_unlock(), that lock must be held across the > > entire read-side critical section in order to avoid deadlock. > > Now that the end of the RCU read-side critical section is > > deferred until sometime after interrupts are re-enabled, this > > requirement could be lifted. However, because the end of the RCU > > read-side critical section is detected sometime after interrupts > > are re-enabled, this means that a low-priority RCU reader might > > remain priority-boosted longer than need be, which could be a > > problem when running real-time workloads. > > > > My current thought is therefore to leave this constraint in > > place. Thoughts? > > > > Anything else that I should be worried about? ;-) > > > > Thanx, Paul > > >
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